Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy into light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a forward bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and extracted to the surrounding environment from all transparent surfaces of the LED.
The human eye is sensitive to variations in color and can, therefore, detect relatively small differences in emission wavelengths of LEDs. Perceptible variations in color emitted by LED devices designed to emit a single color of light can reduce customer satisfaction and reduce overall acceptance of the device for commercial uses. To ensure that LED devices emit light within an acceptable color range, white and color LEDs used in manufacture of the devices can be tested and sorted by color or brightness into different bins, generally referred to in the art as binning. Each bin typically contains LEDs from one color and brightness group and is typically identified by a bin code. White emitting LEDs can be sorted by chromaticity (color) and luminous flux (brightness). Color LEDs can be sorted by dominant wavelength (color) and luminous flux (brightness), or in the case of certain colors such as royal blue, by radiant flux (brightness). LEDs can be shipped, such as on reels, containing LEDs from one bin and are labeled with the appropriate bin code.
LED binning is an effective method of providing LED devices having a light output of the desired color. However, the LED emission properties can be altered by various factors of the LED operating environment. Temperature fluctuations, for example, can alter LED emission properties resulting in a change in the color of light emitted from an LED device.
In order to address the alteration of LED emission properties, LED devices can incorporate one or more optical sensors to monitor the color of the emitted light. Changes in the color of emitted light are detected by the one or more optical sensors and a correction is made to return the emitted light to the original color setting of the LED device. As a result, LED devices incorporating optical color control apparatus can respond to dynamic operating conditions to provide a constant color of emitted light.
Optical color control systems, nevertheless, present several disadvantages. Optical color control systems, for example, increase the complexity and manufacturing cost of LED devices. Moreover, optical color control systems are often designed according to the specifications of the luminaire or fixture in which the LED devices will be disposed, thereby precluding LED device portability to various luminaire or fixture constructions. Optical control systems are additionally closed loop systems requiring a zeroing step to set the system to the correct emission parameters for controlling the color of emitted light. A zeroing step contributes to increases in manufacturing time and cost.